1. Field of the Invention
The present invention relates to an organic electroluminescent device. Particularly, it relates to a thin film type device which emits light when an electric field is applied to a luminescent layer comprising an organic compound.
2. Discussion of Background
Heretofore, it has been common that thin-film type electroluminescent (EL) devices are made of an inorganic material which is obtained by doping a semiconductor of a Group II-VI compound such as ZnS, CaS or SrS with Mn or a rare earth element (such as Eu, Ce, Tb or Sm) as the luminous center. However, the EL devices made of such inorganic materials, have the following problems:
1) Alternate current drive is required (usually from 50 to 1000 Hz),
2) The driving voltage is high (about 200 V),
3) It is difficult to obtain full coloring particularly blue color, and
4) Costs of peripheral driving circuits are high.
However, in order to overcome the above problems, there have been activities, in recent years, to develop EL devices using organic thin films. Particularly, the luminous efficiency has been improved to a large extent over conventional EL devices employing a single crystal of e.g. anthracene, by the development of an organic electroluminescent device wherein the electrode material has been optimized for the purpose of improving the efficiency for carrier injection from the electrode in order to increase the luminous efficiency, and a hole transport layer made of an aromatic diamine and a luminescent layer made of an aluminum complex of 8-hydroxyquinoline, are provided (Appl. Phys. Lett., vol. 51, p.913, 1987).
In addition to the above electroluminescent device employing a low molecular weight material, with regard to the material of a luminescent layer, an electroluminescent device employing a conjugated polymer material such as poly(p-phenylenevinylene), poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene], poly(3-alkylthiophene) or polyfluorene, has been developed, and a device having a low molecular weight luminescent material and an electrontransport material mixed and dispersed in a polymer such as polyvinyl carbazole, has been developed.
Here, a critical issue in the organic electroluminescent device is decrease in the driving voltage. For example, a low voltage drive from a battery is required in a display device of a portable equipment, and even for general application other than portable equipment, the cost of a drive IC depends on the driving voltage, and the lower the driving voltage, the lower the cost.
Accordingly, in order to improve the contact between an anode and a hole transport layer, it has been studied to form a hole injection layer between the two layers to decrease the driving voltage. The material to be used for the hole injection layer is required to have good contact with the anode, to be able to form a uniform thin film, to be thermally stable i.e. to have a high melting point and a high glass transition temperature (Tg), preferably a melting point of at least 250xc2x0 C. and a glass transition temperature of at least 85xc2x0 C. Further, it is required that the ionization potential is low so that the hole injection from the anode is easy, and the hole mobility is large. Various materials for the hole injection layer have been studied, and e.g. porphine derivatives and phthalocyanine compounds (JP-A-63-295695), star burst type aromatic triamines (JP-A-4-308688), organic compounds such as polythienylenevinylene, polythiophene and polyaniline, sputtered carbon films and metal oxides such as vanadium oxide, ruthenium oxide and molybdenum oxide have been reported.
Further, a conjugated polymer having no electron-accepting compound mixed therewith is used for the hole injection layer in some cases (JP-A-4-145192), however, the driving voltage is so high that a luminance of from 3000 to 5000 cd/m2 is obtained at a driving voltage of from 28 to 35V.
In order to overcome the above problems, it is attempted to mix a small amount of an electron-accepting compound with the hole transport compound. For example, it is disclosed that a low voltage drive can be achieved by mixing TBPAH (tris(4-bromophenyl)aminium hexachloroantimonate) as the electron-accepting compound with a non-conjugated hole transport polymer (JP-A-11-283750). However, TBPAH undergoes thermal decomposition during vapor deposition, and thus its addition to the hole injection layer by co-vapor deposition is inappropriate. Accordingly, it is usually mixed with a hole transport material by coating, however, since the compound has low solubility in a solvent, and it is not suitable for coating method. Further, this electron-accepting compound has a bromine atom, and such is considered problematic in view of long-term stability also.
Further, it is disclosed that the hole transport compound is doped with FeCl3 as the electron-accepting compound by vacuum vapor deposition (JP-A-11-251067), however, this electron-accepting compound is corrosive, thus causing a damage over a vacuum deposition apparatus.
Such a high voltage at the time of driving and low stability including heat resistance of an organic electroluminescent device are serious problems as a light source of e.g. a facsimile, a copying machine or a back light of a liquid crystal display, and such are undesirable particularly as a display device of e.g. a full color flat panel display.
Further, indium tin oxide (ITO) which is commonly used as an anode in a conventional organic electroluminescent device has a surface roughness (Ra) at a level of 10 nm, and in-addition, it has protrusions locally in many cases, thus causing short-circuit defects at the time of device fabrication.
Under these circumstances, it is an object of the present invention to overcome the above conventional problems, and to provide an organic electroluminescent device which can be driven at a low voltage with a high luminous efficiency, which has a favorable heat resistance and which can maintain stable luminous properties over a long period of time.
Further, it is an object of the present invention to provide an organic electroluminescent device which can prevent short-circuit defects at the time of device fabrication resulting from the above-described surface roughness of the anode.
The organic electroluminescent device of the present invention, having an anode, a cathode and a luminescent layer interposed between the two electrodes on a substrate, is characterized by that a layer containing an electron-accepting compound containing a boron atom represented by the following formula (I) and a hole transport compound is formed between the luminescent layer and the anode: 
wherein each of Ar1 to Ar3 which are independent of one another, is an aromatic hydrocarbocyclic group or aromatic heterocyclic group which may have a substituent.
Namely, the present inventors have conducted extensive studies to overcome the conventional problems and to provide an organic electroluminescent device capable of being driven at a low voltage, and as a result, they have found that the above problems can be overcome by forming, in an organic electroluminescent device having an anode, a cathode and a luminescent layer interposed between the two electrodes, on a substrate, a layer containing an electron-accepting compound containing a boron atom represented by the above formula (I) and a hole transport compound between the luminescent layer and the anode. The present invention has been accomplished on the basis of this discovery.
In the present invention, use of a hole transport compound and a specific electron-accepting compound together made it possible to improve the luminous properties and the heat resistance of the device simultaneously. Namely, by mixing the above specific electron-accepting compound containing boron with a hole transport compound, charge transfer takes place, and as a result, holes as free carriers are formed to increase the electric conductivity of the layer containing the compounds. By forming such a layer, electrical connection between the luminescent layer and the anode improves, the driving voltage decreases, and at the same time, stability at the time of continuous driving is improved. Further, particularly in a case where a hole transport compound having a glass transition temperature of at least 85xc2x0 C. is used, the heat resistance of the device is significantly improved also.
Further, the electron-accepting compound containing boron represented by the above formula (I) used in the present invention has a high electron acceptability, and at the same time, has subliming properties, and it can thereby be used for layer formation by vacuum vapor deposition. Further, it has a high solubility in a solvent, and it can thereby be used for layer formation by a wet coating method, and the degree of freedom in the device design can be increased.
Here, the layer containing a hole transport compound and an electron-accepting compound according to the present invention is a layer having hole transport properties. It may be formed at any part so long as it is between the anode and the luminescent layer, and its formation is not limited to direct formation on the anode as illustrated in FIGS. 1 to 3. However, it is most advantageous to form the layer to be in contact with the anode so as to make adequate use of merits of this layer such as a good electrical connection with the anode (inorganic material) and a high heat resistance.
In such a case, by forming a layer containing a polymer material as the base on the anode by a coating process, such an effect is also obtained that the above-described surface roughness of the anode is decreased, a favorable surface smoothing effect can be obtained, and short-circuit defects at the time of device preparation can be prevented.
In the present invention, as the hole transport compound, the following are preferred:
(1) an aromatic tertiary amine compound or a compound having an aromatic tertiary amino group, preferably a compound containing an aromatic tertiary amino group and having a molecular weight of from 1,000 to 1,000,000,
(2) a porphine derivative or a phthalocyanine derivative, or
(3) a conjugated polymer.
In the present invention, the content of the electron-accepting compound in the layer containing the hole transport compound and the electron-accepting compound is preferably from 0.1 to 50 wt % based on the entire material forming the layer containing it.